This invention relates to equipment for optimizing the color purity tolerance of color television picture tubes and particularly to an improvement whereby the optimized tolerance includes compensation for the earth's magnetic field, thermal expansion of the shadow mask, as well as a preselected safety factor.
The three beams of a color television picture tube must be adjusted in position for several reasons to enable a satisfactory picture to be reproduced on the viewing screen of the tube. Adjustment for color purity is required with all color picture tubes. The purity adjustment provides for the beams to land only on their respective color phosphor elements. Obviously, if the displayed picture lacks purity, the red beam, for example, might land on green or blue phosphors and result in a false color scene reproduction.
In a non-matrix type of color picture tube, the beam portion passing through an aperture of the shadow mask is smaller than the individual phosphor element on the viewing screen so that when it is properly landed on the desired phosphor element, it will not illuminate the adjacent different color elements. In a matrix-type of picture tube, in which dark guard bands separate adjacent different color phosphor elements, the beam portion passing through an aperture may be larger than the phosphor elements and still result in color purity. In both types of picture tubes, it is desirable to center the respective beams on their phosphor elements to minimize the possibility of a loss of purity if the beams are undesirably moved due to mechanical changes of the picture tube or stray magnetic fields.
It is generally recognized that purity may be controlled at the center portion of a viewing screen by varying the position of two magnetized purity rings mounted for rotatable motion about the neck of the picture tube. Purity is controlled at the edge regions of the picture tube by axial movement of the deflection yoke which moves the deflection center of the beams and hence controls their landing positions at portions away from the center of the viewing screen.
One method of adjusting color purity, using the purity rings and the deflection yoke, is disclosed in U.S. Pat. No. 3,916,437 issued to Barbon. Although this is an effective method for adjusting purity at the tube factory, misregister problems may arise after the picture tube has been installed in the television receiver. These problems are typically caused by the magnetic field environment present in the receiver which was not experienced during factory adjustment. These problems become particularly troublesome and expensive in those cases where the deflection yoke and purity rings are permanently immobilized after purity adjustment at the tube factory. In these situations, the receiver manufacturer must either install relatively expensive compensating circuitry in the receiver, or return the tube to the tube manufacturer taking the risk that the replacement tube may also develop purity errors.
One approach to solving these misregister problems involves simulating the magnetic environment of the receiver by placing pieces of metal at various locations around the deflection yoke. The size and location of the metal pieces are determined by trial and error until the misregister of the tube in the test system duplicates the misregister in the receiver. Since each type of receiver produced by each receiver manufacturer typically has its own unique magnetic field environment, it is apparent that simulating all possible receiver fields using pieces of metal is expensive, tedious, time consuming and may not always be possible.
Another approach is described in copending U.S. patent application, Ser. No. 783,215, filed Mar. 31, 1977, entitled "Method and Apparatus for Simulating Magnetic Environment of Television Receivers," inventors William Arthur Sonntag and Theodore F. Simpson. That method comprises placing a deflection yoke and color purity adjustment apparatus in operating positions relative to a multi-beam color television picture tube. In addition, a set of auxiliary coils are placed on opposite sides of the picture tube. The tube is then operated to cause a raster to be scanned on the viewing screen of the tube. The auxiliary coils are energized to cause the beams to be deflected from their nominal beam landing sites by a predetermined amount in a predetermined direction within each of at least two predetermined regions within the raster area. These predetermined deflections are chosen to approximate deflections which are likely to be caused by the magnetic environment present in the television receiver. Color purity of the tube is then adjusted using a prior art method such as that taught by Barbin in U.S. Pat. No. 3,916,437.
The method and apparatus of the above referenced copending application, although effective in compensating for receiver induced purity errors, does not account for other sources of purity errors, such as thermal expansion of the shadow mask, the effect of changes in strength and/or direction in the ambient magnetic field, and other less well-defined sources of error.